How to correctly determine what creates a long task and increases tbt?

Question

Web development

I understand that you need to see the trace in the performance tab. But she is completely uninformed.
Here I click on the first long task, and there it is.


What is system (self,children) is completely incomprehensible.

Bottom up (In which they are called to watch) or call tree do not give anything either. Generally.
Something like you can sometimes fish out something from the event log.



But here, for example, what kind of request, what exactly paint, what style was recalculated, etc.